Regional and Individual Differences in Cutaneous Sensitivity to Chemical Irritants: Capsaicin and Menthol

Regional and individual differences in sensory irritation produced by capsaicin and l-menthol were assessed in two experiments. Thresholds for irritation were measured on the forearm and cheek using a psychophysical method that combines a bilateral, forced-choice detection task with an intensity scaling task. Experiment 1 showed that thresholds on the cheek were higher than on the forearm for both chemicals; on average, thresholds were much lower for capsaicin than for menthol; there were large individual differences in thresholds for both chemicals on both sites; and capsaicin and menthol produced different qualities of sensation. In contrast to earlier findings, capsaicin failed to produce itching on either site. Experiment 2 showed that itching could be evoked on the forearm, but not the face, when capsaicin was applied in a single dose rather than in an ascending concentrations series, as it had been in experiment 1. It was hypothesized that the absence of itching in experiment 1 was due to a more rapid desensitization for itching compared to burning and stinging, and that the lack of itching on the cheek may reflect different chemosensory innervation on facial and forearm skin. Overall, the results demonstrate that psychophysical measurements can provide information about regional and individual differences in the susceptibility to sensory irritation from contact with environmental chemicals.